The isolation of the lumenal plasma membrane from the mammalian urinary bladder is being carried out by detachment of the surface membrane resulting from its exposure to thioglycolate and purification by centrifugation on a discontinuous sucrose gradient. Improvement in purity and yield of this procedure, previously described by Chlapowski et al., is in progress with the goal of achieving a biochemical analysis of the lipids present (which may account for the impermeability of the membrane to water) and of the characteristic protein particles that are assembled into plaques within the membrane. The particles, once isolated in pure form, will be used as antigens for immunochemical staining of bladder epithelium in order to determine whether they may be assembled and integrated into membrane within the Golgi region. If time permits, a second system, the epididymal epithelium, will be used as a source from which to isolate the Golgi apparatus by centrifugation of homogenates. In this tissue the size and enzymatic composition of this membrane system can be controlled by testosterone (i. e., castration and hormone replacement). The isolated Golgi apparatus can then be analyzed biochemically in various functional states, particularly with reference to its role in the assembly of specialized areas of the cell surface.